The present invention relates in general to a driving technique for stabilizing an optical output in an optical data signal generating unit used in an optical communication system and, more particularly, to an apparatus suitable for data burst transmission which effectively performs temperature compensation for an output level of burst light radiated from a semiconductor light-emitting element.
The importance of optical communication systems in achieving data transmission between a plurality of work stations has been increasing. An optical star network system has been developed as one type of an optical communication network system. A digital optical signal exchange between the work stations connected in a system of this type is performed through a burst signal in a time division manner. The burst signal is transmitted from a specific sending work station to a receiving work station within a predetermined period which is less than 1 msec as assigned by time division. For example, the sending station may transmit data having a frequency of 10 MHz or higher to the receiving station. In general, most optical communication network systems have optical signal generators each of which comprises a semiconductor laser whose luminous efficiency is very sensitive to changes in temperature. Therefore, a new problem is presented by such optical communication network systems, although this problem does not occur in conventional electrical signal transmission systems using a coaxial cable. The first problem to be solved is variations in a burst optical output which are caused by heat generation from the semiconductor laser during the communication or burst period. The luminuous efficacy of a semiconductor light-emitting element including a semiconductor laser diode is degraded due to heat generation from the element during the burst period. In the laser diode, the threshold current level is increased due to heat radiation therefrom, and an optical output from the laser diode is degraded. As a result, this leads to a crucial problem wherein a thermal sag phenomenon occurs in the optical communication system.
At present, in order to compensate for an optical output change due to a temperature change, an automatic power control (APC) is arranged in the optical communication system. The APC is a type of analog control. According to the APC technique, a bias current flowing in the laser diode is increased or decreased in response to changes in temperature, but is controlled to be kept close to the threshold current level (bi-directional APC). However, the conventional optical output stabilizing arrangement has a slow time response as compared with the high-speed of the burst signal transmission. In order to apply the conventional optical output stabilizing arrangement to data burst transmission, a feedback loop response of a burst signal having a frequency of several megahertz must be achieved. In order to improve the response speed, ultra-high-speed active devices must be used which have a high cost, the apparatus as a whole expensive. In addition to the above drawback, the conventional optical output stabilizing technique presents the following worse problems. Since two types of feedback operation modes (i.e., increase and decrease) are used to set the drive current or bias current of the laser diode to be closer to the target value, oscillation in response to excess phase transition caused by the delay factors of the circuit elements, such as photodetectors and transistors, occurs. As a result, the compensation efficiency is degraded.